1. Field of the Invention
The present invention relates to a method of estimating a vehicle speed of a vehicle, comprising estimating the vehicle speed on the basis of a wheel speed and comparing a reference threshold derived from the estimated vehicle speed with the wheel speed to control the supplying of hydraulic braking pressure to wheel brakes.
2. Description of the Prior Art
Such a method of estimating a vehicle speed has been known, for example, from Japanese patent publication No. 6902/81.
In the above prior art, a wheel speed is compared with an estimated vehicle speed, and when the wheel speed is larger than the estimated vehicle speed, the wheel speed is used as an estimated vehicle speed. When the wheel speed is smaller than the estimated vehicle speed, a speed corresponding to -0.8 to -1.1G is subtracted from the estimated vehicle speed until the wheel speed has been equal to such estimated vehicle speed and the obtained value is used as a new estimated vehicle speed.
Such a characteristic can be illustrated as shown in FIG. 6. That is, troughs of a wheel speed Vw are connected with a predetermined gradient, so that a tentative vehicle speed Vv is estimated.
With such an estimating method, however, the following problem arises: The load of wheel contacting the ground varies during travelling of a vehicle on a rough or bad road and hence, the braking force also varies during braking even if depressing force on a brake pedal is constant. For this reason, a longitudinal pulsation is generated in a suspension and causes a pulsation in the speed of a wheel which is rotating in contact with the ground. With this pulsation produced in the wheel speed, an estimated vehicle speed Vv goes above an actual vehicle speed Vv* as shown in FIG. 7. As a result, a condition is apt to appear that a wheel speed Vw falls below a reference threshold derived from an estimated vehicle speed Vv.